1. Field of the Invention
The present invention pertains to a truly erasable writing medium that can be satisfactorily dispensed through a ballpoint writing instrument to give a mark that can be removed from the writing surface with comparative ease by the use of a substantially non-abrasive eraser within the first few hours after being applied to the writing surface but which thereafter sets up to produce a permanent non-erasable line.
2. Description of the Prior Art
Many frequently interrelated factors must be taken into account in the formulation of a writing medium. These factors fall into two categories, namely, those which stem from the type writing instruments to be used in dispensing the writing medium onto the writing surface, and those which are concerned with the desired characteristics that the writing medium possesses after being dispensed.
Writing instruments available to the formulator of a writing medium include ballpoint pens, variations of the so-called soft or porous-type pens, and fountain pens. In some instances, such as is the case with lead pencils or crayons, the writing medium itself is in effect the dispensing means.
Insofar as the characteristics of the writing medium in the "after dispensed" state are concerned, appearance and relative indelibility are the most important. In most cases, the appearance of the writing medium means little more than the color of the written line. Indelibility is related to the ease of removal of the written line. Thus, an indelible writing medium is one which is resistant to removal from the writing surface. Obtainment of a high degree of indelibility is not always necessarily desirable. In some instances one may want a writing medium which is not easily removable by mechanical means from the writing surface, while in other instances one may want a writing medium which is easily removable. A writing medium having this latter attribute is generally referred to as "erasable" writing medium, and if such a medium is to be considered as being truly erasable, it must be readily removable from the substratum from which it has been applied without any damage of any significant degree to the area of the substratum involved.
In order to understand the nature of this invention, consideration must first be given to the conditions which the formulator of an erasable writing medium must avoid. This in turn requires at least an elementary knowledge of the structure of paper, since paper is the substratum most commonly employed as a writing surface.
Paper is essentially a mat of randomly oriented cellulose fibers. Thus, paper consists of solid structural members, namely the cellulose fibers, having numerous minute voids therebetween. From this description, it may readily be seen that the formulator or designer of an erasable writing medium must prevent the colorant portion of the medium from penetrating to any substantial degree into the voids in the surface being written upon because if there is substantial penetration into the voids the subsequent removal of the colorant by mechanical means can not be accomplished without damage to the writing surface. Likewise, it may readily be seen that for a writing medium to be erasable, the colorant particles must be prevented from affixing themselves with any substantial degree of permanence, either by chemical reaction or as a result of mere physical attraction, to the solid members of the paper substratum.
In the past, attempts to achieve a truly erasable writing medium for a ballpoint pen usually have been unsuccessful. These attempts have employed two somewhat overlapping approaches to solving this problem.
In one approach, reliance has been placed upon the ability of a superabrasive eraser to remove ordinary ball pen inks from the paper substratum normally used for writing purposes. As discussed above, such substratum is porous to some degree and the ball pen inks in ordinary usage tend to penetrate those pores. In addition, the colorants in such inks tend to affix themselves to the fibers which constitute the solid portion of the substratum. Therefore, the only way to erase markings made with ordinary ball pen inks is to physically remove a substantial number of the fibers in the vicinity of the markings. As a result, the paper substratum is generally so damaged during the erasure process that it is rendered unsuitable for any further use as a writing surface.
In the other approach to an erasable writing medium, attempts were made to modify the ink so as to prevent the colorant portion thereof from penetrating the pores of the paper substratum. These attempts consisted of substituting pigment-type colorants for the dye-type colorants usually used in ball pen inks. The reasoning behind this approach was that since pigment-type colorants normally have a greater particle size than dye-type colorants, the pigment-type colorants would become substantially immobile upon being deposited upon the surface of the paper substratum and, therefore, would not tend to penetrate into the pores of the paper. However, this line of reasoning overlooked the fact that in a ballpoint writing instrument, the ink is dispensed through a very minute clearance between the ball and socket and that any pigment-type particle which is small enough to be so dispensed will tend to behave as if it were a dye-type particle. Therefore, the colorant portion of such inks was not immobilized to any significant degree and as a consequence, no substantial advantage was gained through substituting the pigment-type colorant for the dye-type colorant insofar as erasability was concerned.
Some success has been achieved in producing erasable ballpoint pen inks that are transitorially erasable, for example, my prior U.S. Pat. No. 4,097,209 hereinafter referred to as Muller et al. and U.S. Pat. No. 3,875,105 to Daugherty et al. Both of these patents teach the use of an elastomer. The Daugherty et al. patent teaches polyvinyl methyl ether and the parent application thereof, Ser. No. 751,759, now abandoned, teaches cis polyisoprene and Natsyn 2200 while my prior Muller et al. patent teaches natural rubber or rubber which essentially duplicates the chemical structure of natural rubber. Both patents disclose the use of a volatile solvent. Daugherty et al. discloses the volatilizing component as being a solvent for the matrix and having an evaporation rate of 3 to 15 on a relative numerical scale on which ethyl ether is assigned an evaporation rate of 1. My prior Muller et al. patent discloses a volatile low boiling organic solvent having a boiling point less than 180.degree. C. and exhibiting 100% evaporation within 60 minutes. In summary, both my prior Muller et al. patent and the Daugherty et al. patent taught an erasable ink that included the following: (1) the use of a pigment as a colorant; (2) a rubber type elastomer polymer; and (3) the use of mixtures of volatile and non-volatile solvents.
It now has been found that the inclusion of a volatile solvent in a ballpoint pen ink is highly undesirable. For example, during the production of an ink containing a volatile solvent it must be produced in sealed mixing equipment and also it is detrimental to the smooth writing quality expected from a ballpoint pen ink because its presence produces a thick ink "build-up" on the point and also contributes to a dry writing unit that does not have ease of application.
The below-listed prior United States patents are made of record herein under 37 C.F.R. 1.56:
______________________________________ 2,715,388 Cofield et al. 2,833,736 Glaser 2,852,397 Goessling 2,852,398 Goessling 2,853,972 Cofield 3,553,710 Lloyd 3,563,782 Liberman et al. 3,834,823 Seregely et al. 3,875,105 Daugherty et al. 3,912,520 Miyajima et al. 3,993,492 Woolly 4,097,290 Muller et al. ______________________________________